1. Field of the Invention
The present invention relates to a lithium-ion secondary battery and a method of charging a lithium-ion secondary battery.
2. Related Background Art
Along with recent dissemination and development of various portable devices, lithium-ion secondary batteries have further been desired to reduce their cost and improve their characteristics. One of characteristics expected to improve is a rapid charging characteristic. Mainly known as conventional methods of charging lithium-ion secondary batteries are constant-current charging, constant-current/constant-voltage charging, and constant-voltage charging (see, for example, Japanese Patent Application Laid-Open Nos. 5-111184, 7-296853, 8-45550, and 5-21093).
The constant-current charging is a method in which the charging voltage is regulated so as to attain a fixed charging current, and the charging of the secondary battery is stopped when the charging voltage reaches a predetermined full-charge voltage (e.g., 4.2 V). In the constant-current charging, the charging efficiency deteriorates under the influence of IR drop and polarization when the charging voltage approaches the full-charge voltage, whereby the charging amount tends to be in short. This tendency becomes remarkable in particular when rapid charging is performed. In the case where such a constant current charging method is used, if the constant-current charging is further carried out to a voltage higher than the full-charge voltage in order to increase the charging amount after the full-charge voltage is attained, an overcharged state may partly occur in the positive electrode or negative electrode in the secondary battery, thereby decomposing electrolytes and generating gases.
In the constant-current/constant-voltage charging, which is normally used for charging lithium-ion secondary batteries, constant-current charging is performed until the charging voltage becomes the full-charge voltage, then the charging is switched to constant-voltage charging, and the charging is terminated when the charging current becomes a predetermined value or lower. This method is more likely to eliminate the shortage in charging amount than is the constant-current charging, but complicates charger circuits, thereby causing the cost to rise.
On the other hand, the constant-voltage charging is a method in which a constant charging voltage is supplied to the secondary battery. When the charging voltage is set appropriately, there is no fear of overcharging the secondary battery. When appropriate charging time and charging stop current value are set, a sufficient charging amount can be obtained, while rapid charging is possible. Also, charger circuits become simpler than those in the constant-current/constant-voltage charging, whereby the cost is expected to decrease.